1. Field of the Invention
This invention relates to a semiconductor device and, in particular, to a semiconductor device having a junction material portion consisting of a parent phase of a low-melting-point junction material and fine particles of a high-melting-point junction material which are uniformly dispersed therein.
2. Description of the Related Art
FIG. 5 is a schematic longitudinal sectional view showing part of a conventional semiconductor device, and FIG. 6 is a perspective view of the junction material portion of the semiconductor device shown in FIG. 5. In these drawings, a semiconductor element 1 is attached to a die pad 3 through a junction material 4, which is, for example, a three-layer structure consisting of a high-melting-point solder layer 4b and low-melting-point solder layers 4a respectively provided on opposite surfaces thereof.
In this semiconductor device, constructed as described above, the semiconductor element 1 is attached to the die pad as follows: First, the junction material 4 is cut up in the same size as the semiconductor element 1 and is placed between the semiconductor element 1 and the die pad 3. Then, the junction material 4 is heated to and kept at a temperature higher than the melting point of the low-melting-point solder 4a and lower than that of the high-melting-point solder 4b, whereby the low-melting-point solder 4a is brought to a molten state, attaching the semiconductor element 1 to the die pad 3. Further, as a result of keeping the junction material at such a temperature, a mutual dispersion occurs between the low-melting-point and high-melting-point solder components 4a and 4b, with the result that they are eventually formed into a single junction material having a uniform composition. The melting point of this junction material thus formed is higher than that of the low-melting-point solder 4a and lower than that of the high-melting-point solder 4b. This melting point can be controlled by varying the compositions and volumes of the low-melting-point and high-melting-point solder components 4a and 4b before melting. In this way, the semiconductor element 1 can eventually be attached to the die pad 3 by means of a single-phase junction material.
Afterwards, the electrode terminals of the semiconductor device 1 and the tips of inner leads (not shown) are connected to each other through fine metal wires by a wire bonding technique such as thermocompression or ultrasonic bonding. Subsequently, the device is subjected to the steps of molding, lead cutting, lead bending, electrical parameter check, etc. before the production thereof is completed.
A problem with the semiconductor device described above is that if the semiconductor device 1 is to be attached to the die pad 3 with the low-melting-point solder 4a wetting the entire back surface of the semiconductor element 1, it is necessary for the junction material 4 to have the same size as the semiconductor device 1, which means each time the size of the associated semiconductor device 1 is changed, the size of the junction material 4 must be changed accordingly. Further, in the conventional junction material 4, the interface between the high-melting-point and low-melting-point solders 4b and 4a is a plane, so that the area utilized for the mutual dispersion is relatively small. Accordingly, the above-mentioned heat treatment must be performed for a long period of time before the junction material attains a desired composition through mutual dispersion.